Soybean is the leading oilseed crop produced and consumed worldwide (Wilcox World distribution and trade of soybean, 2004, Soybeans: Improvement, production, and Uses, 3rd ed., Agron. Monogr. 16, ASA, CSSA, and SSSA, Madison, Wis. p: 1-14; Hymowitz, 2004, Speciation and Cytogenetics, p. 97-136. In H. R. Boerma and J. E. Specht (ed) Soybeans: Improvement, production, and Uses. 3rd ed. Agron. Monogr. 16. ASA, CSSA, and SSSA). In the past half century, the USA has been the world's leading producer. In 2003, the USA produced 35% (65.8 million metric tons (MT)) of the world's total soybean (FAOSTAT, 2004, Production Crops). Soybean has many insect pests limiting its production in other parts of the world, including the soybean aphid.
Around 2000 A.D., the soybean aphid (Aphis glycines Matsumura) a native to eastern Asia, became a major sucking pest of soybean [Glycine max (L.) Merr.] in North America. Since then, this insect pest has rapidly spread to the major soybean production areas in the USA and Canada (Plant Health Initiative, 2004, Soybean Aphids Research Update “Aphids discovered in Wisconsin” from the North Central Soybean Research Program (NCSRP) published online by the Plant Health Initiative Available at planthealth.info/soyaphid.htm (verified Oct. 5, 2004). Aphid outbreaks have been severe in the northern part of the Midwestern USA and in Ontario, particularly in years 2001 and 2003.
Several factors affect soybean aphid outbreaks, including environmental conditions, over-wintering success, cultural practices, natural enemies, and the synchronization of soybean and aphid development (Wu et al., 1999, How Peasants Can Increase Wealth [Nongmin zhifu zhiyou] 6:20). The soybean aphid develops large colonies on soybean plants. Plant damage occurs when large numbers of aphids remove significant amounts of water and nutrients as they feed (suck) on leaves and stems, causing leaves to wilt, curl, yellow, and even drop off. Other symptoms of direct feeding damage include plant stunting, poor pod fill, reduced pod and seed counts, smaller seed size, and nutrient deficiencies resulting in overall yield and quality reduction (DiFonzo and Hines, 2002, Michigan State University Extension Bulletin E-2746). Significant yield loss (8-25%) occurs when the aphid heavily infests the soybean plants during the early reproductive stage (DiFonzo and Hines, 2002, Michigan State University Extension Bulletin E-2746). Honeydew, a sticky substance excreted by soybean aphids onto the leaves, leads to the development of sooty mold, which affects photosynthesis and results in yield loss (Baute, 2004, (Soybean Aphid Fact sheet and Soybean Webpage sponsored by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), published online). During the feeding process, soybean aphids are capable of transmitting viruses including alfalfa mosaic virus, soybean mosaic virus, and bean yellow mosaic virus. These viruses commonly occur together and form a disease complex that leads to plant stunting, leaf distortion and leaf and stem mottling, reduced pod numbers, and seed discoloration (Glogoza, 2002, North Dakota State University Extension Bulletin E-1232).
Aphids are particularly difficult to control because of their rapid reproduction rates and ability to disperse over wide areas. Populations build rapidly (females give live birth, young mature in 3-7 days, doubling time 2-5 days under favorable conditions). Winged forms appear and disperse to other fields under high insect densities and when infested plants are stressed. Since aphids are relatively weak fliers, long-distance dispersal is often at the mercy of prevailing winds.
Aerial applicators frequently report having to stop to clean their windshields from flying into clouds of these aphids above heavily infested fields. In 2001, the influx of winged soybean aphids into the open dome of the Toronto Blue Jays even caused an early end to a Toronto Blue Jays game. It's these flights that lead to rapid, progressive colonization of soybean, almost like a wave moving across the countryside. Under favorable conditions for aphid infestations, the settling of winged aphids into uninfested fields has been described as “aphid rain.” (Ostlie, Soybean Aphid Pages published online by Just for Growers, MN (University of Minnesota) Soybean Production, published online by the University of Minnesota, the University of Minnesota Extension Service, and the MN Soybean Research and Promotion Council Jul. 6, 2004).
Insecticides are the primary available method of controlling soybean aphids in the USA. Although the use of insecticides can be a quick and easy way to control aphids, the ideal time to spray is not well defined. Insecticides also have many adverse effects such as killing beneficial insects, environmental contamination, and increased production costs (Sun et al., 1991, Soybean Sci. 10(2):98-103). Aphid populations may resurge when applications of insecticides are poorly timed or applied.
In the USA, there are currently no commercial soybean cultivars with aphid resistance and there are no reported resistance sources in early maturing soybean germplasm. Although there have been recent reports of aphid resistant soybean plants, (Hill et al., J. of Econ. Entomol. 97:1071-1077 (2004); Hill et al., Crop Sci. 44:98-106 (2004); Mueller, et al., The 2003 Entomological Society of America Annual Meeting and Exhibition Cincinnati, Ohio, October 2003, all of which are herein incorporated by reference), with further reports showing the results of studies on their effectiveness, (Li et al., J Econ Entomol. 2004 June; 97(3):1106-1111). These soybean plants are late maturing and not well suited for commercial development.
Currently none of the commercial soybean varieties show resistance to the aphid and further there are no sources of resistance reported in early maturing soybean germplasm in the USA. Therefore, developing soybean varieties that are resistant to aphids is a long-term solution to the aphid problem. Furthermore, there is a need for soybean plant varieties with engineered resistance to specific aphid biotypes in addition to the desirability of having commercially available soybean plants with a broad resistance to aphids.